The present invention relates to improvements in coating mechanisms such as for applying coating to a traveling paper web and particularly to an improved apparatus and method which makes it possible to simultaneously coat both surfaces of a web and makes it possible to apply a sizing to one surface and a coating to the other surface.
The coating of webs, such as paper, paperboard, woven and nonwoven fabrics and so forth, have been carried out in the past employing a variety of different mechanisms and procedures. It has not been common practice to be able to apply a coating to both surfaces of a traveling web essentially simultaneously, although one such attempt is disclosed in U.S. Pat. No. 3,489,592, Wallsten. The structure shown in this patent requires the limitations of a downrunning arrangement with opposing blades, and the structure shown is not capable of sophisticated high speed operation which can apply various types of coatings to obtain a finished paper. One particular problem has been to be able to apply a sizing to one surface of a traveling paper web and apply a coating to the other surface. Broadly speaking, sizing employs starches whereas coating uses pigment such as clay. It will be understood that while the present invention attains particular advantages in the application of a sizing to one surface of a paper web and of a coating to the other surface, the features of the invention may be employed in other ways such as by applying two different forms of coating to opposite sides of the web, although in some cases, the same coating may be applied to the opposite sides of a web. Thus, a feature of the present invention is its capability to handle varying types of coatings and to apply them to both sides of a traveling web essentially simultaneously so that the web is coated in a single operation.
It is another feature of the invention to coat both sides of a traveling web of paper and utilize a sophisticated improved coating structure for the outer surface of the web, such as disclosed in U.S. Pat. No. 3,348,526, Neubauer. This structure utilizes a trailing knife or blade mounted with its working edge pressed against the web where the web passes over the roll. The knife functions to spread the coating composition applied to the web and at the same time meters the material whereby uniform film results. The present invention makes it possible to use a similar type of coating apparatus operating in a different manner to coat the opposite side of the web metering the coating to be applied. Further, there is a coaction between the coating methods applied whereby the coating to each surface is applied in an improved manner obtaining new and unforeseen results in an improved coated paper product.
In a specific embodiment of the invention, a back-up roll is provided in the coater, preferably one with a resilient covering and the outer surface of the web is engaged and coated by an inverted blade or knife mounted in the coater beside the back-up roll with its working edge and the surface of the roll defining a nip in the coater. The blade defines an acute angle on the feed side of the nip with respect to a plane passing tangentially of the surface of the roll at the nip. An extrusion device is mounted on the coater in a position where it is operable to extrude a ribbon of coating material continuously onto the blade adjacent its working edge and the nip defined thereby. The coating material on being deposited on the blade is displaced upwardly along the blade and into the nip by the coating material being continuously extruded from the extrusion device. As the back-up roll is rotated and the web is advanced through the nip, the coating material which fills the nip is spread out in a smooth film on the web being treated. Only enough coating material is extruded so as to produce an excess of coating material at the nip, and excess material is troweled off by the blade during the coating operation then to be caught in a pan or other container once the material may be recirculated to the extrusion device. A structure of this type is shown and disclosed in U.S. Pat. No. 3,348,526, and the present arrangement additionally in a preferred embodiment applies a coating to the back-up roll at a location in advance of where the web is laid on the back-up roll. The coating is applied by a suitable mechanism, such as a trailing blade coater which carefully meters a layer of coating in a uniform layer across the width of the back-up roll. As the coating is carried up into a coating zone, where the web is laid onto the back-up roll, it is transferred off the back-up roll onto the web. This transfer begins immediately as the web contacts the roll, but a unique effect is attained by the hydraulic pressure generated between the back-up roll and the blade. That is, the coating on the back-up roll is still liquid as it passes between the web and the back-up roll, and the coating on the opposite surface of the web is also liquid where it is pressed against the web by the trailing blade coater. The two liquids on the opposite side of the web generate a hydraulic pressure which will tend to be uniform along the length of the blade, and the web in effect floats between two hydraulic layers, with one layer being the coating between the roll and the web and the other layer being the coating between the blade and the web. This hydraulic bath or float brings the web to a neutral position, neutralized by the pressures of the two coatings so that the coating is more uniformly and more evenly applied on both sides of the web. This reaction enhances the operation of the particular type of trailing blade coater which receives its coating by extrusion ahead of the blade edge so that it operates better to provide a better coated product.
The coaction between the two layers of coating also improves the transfer of coating from the back-up roll to the web in that there are no extreme pressure points generated, but the pressure between the inner surface of the web and the back-up roll is essentially uniform across the working length of the roll so that the transfer pressure is essentially uniform. This helps meter a more uniform layer of coating to the undersurface of the web. This also makes possible a more solid surface back-up roll, and it is not as essential to provide a resilient surface on the back-up roll to coact with the blade coater on the undersurface because the hydraulic support or float of the layer of coating between the web and the back-up roll provides the resiliency which is necessary.
Further because a uniform hydraulic resistance is occurring on both sides of the web at the same location, penetration of the coating on the outer surface and of the sizing on the undersurface is more likely to be uniform, and the coating and the sizing are more likely to remain on the surface rather than migrate at an undesirable distance into the moving web. Such an effect, of course, conserves coating and provides a coating layer on the outer surface of the web which is more suitable.
An object of the invention is to provide an improved coating mechanism for achieving an improved coaction between the coating applicator and the traveling paper web for an improved product.
A further object of the invention is to provide an improved coater capable of simultaneously applying a sizing to one surface of a traveling web of paper and a coating to the opposite side or applying coatings of different characteristics to the two sides of a traveling paper web in substantially the same operation.
A still further object of the invention is to provide an improved coating apparatus and a method of applying coating utilizing hydraulic forces which are present by liquid coating on both sides of the paper with the coating being simultaneously applied to the paper.
Other objects, advantages and features will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments in the specification, claims and FIGURE, in which: